Cisco Discovery Protocol
Encyclopedia
The Cisco Discovery Protocol (CDP) is a proprietary
Data Link Layer
network protocol developed by Cisco Systems
. It is used to share information about other directly connected Cisco equipment, such as the operating system
version and IP address
. CDP can also be used for On-Demand Routing
, which is a method of including routing information in CDP announcements so that dynamic routing protocol
s do not need to be used in simple network.
Cisco devices send CDP announcements to the multicast
destination address
01-00-0c-cc-cc-cc, out each connected network interface. These multicast packets may be received by Cisco switches and other networking devices that support CDP into their connected network interface. This multicast destination is also used in other Cisco protocols such as VTP
. By default, CDP announcements are sent every 60 seconds on interfaces that support Subnetwork Access Protocol
(SNAP) headers, including Ethernet
, Frame Relay
and Asynchronous Transfer Mode
(ATM). Each Cisco device that supports CDP stores the information received from other devices in a table that can be viewed using the show cdp neighbors command. This table is also accessible via snmp. The CDP table information is refreshed each time an announcement is received, and the holdtime for that entry is reinitialized. The holdtime specifies the lifetime of an entry in the table - if no announcements are received from a device for a period in excess of the holdtime, the device information is discarded (default 180 seconds).
The information contained in CDP announcements varies by the type of device and the version of the operating system running on it. This information may include the operating system
version, hostname
, every address (i.e. IP address
) from all protocol(s) configured on the port where CDP frame is sent, the port identifier from which the announcement was sent, device type and model, duplex
setting, VTP
domain, native VLAN, power draw (for Power over Ethernet
devices), and other device specific information. The details contained in these announcements is easily extended due to the use of the type-length-value
(TLV) frame
format. See external links for a technical definition.
Hewlett-Packard
removed support for transmitting CDP from HP Procurve products shipped after February 2006 and all future software upgrades. Receiving and processing CDP information is still supported. CDP support was replaced with IEEE 802.1AB Link Layer Discovery Protocol
, an IEEE standard that is implemented by multiple vendors and is functionally similar to CDP.
Proprietary protocol
In telecommunications, a proprietary protocol is a communications protocol owned by a single organization or individual.-Enforcement:Proprietors may enforce restrictions through patents and by keeping the protocol specification a trade secret...
Data Link Layer
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
network protocol developed by Cisco Systems
Cisco Systems
Cisco Systems, Inc. is an American multinational corporation headquartered in San Jose, California, United States, that designs and sells consumer electronics, networking, voice, and communications technology and services. Cisco has more than 70,000 employees and annual revenue of US$...
. It is used to share information about other directly connected Cisco equipment, such as the operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
version and IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
. CDP can also be used for On-Demand Routing
On Demand Routing
On-Demand Routing is an enhancement to Cisco Discovery Protocol , a protocol used to discover other Cisco devices on either broadcast or non-broadcast media....
, which is a method of including routing information in CDP announcements so that dynamic routing protocol
Routing protocol
A routing protocol is a protocol that specifies how routers communicate with each other, disseminating information that enables them to select routes between any two nodes on a computer network, the choice of the route being done by routing algorithms. Each router has a priori knowledge only of...
s do not need to be used in simple network.
Cisco devices send CDP announcements to the multicast
Multicast
In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
destination address
Multicast address
A multicast address is a logical identifier for a group of hosts in a computer network, that are available to process datagrams or frames intended to be multicast for a designated network service...
01-00-0c-cc-cc-cc, out each connected network interface. These multicast packets may be received by Cisco switches and other networking devices that support CDP into their connected network interface. This multicast destination is also used in other Cisco protocols such as VTP
VTP
VLAN Trunking Protocol is a Cisco proprietary protocol that propagates the definition of Virtual Local Area Networks on the whole local area network. To do this, VTP carries VLAN information to all the switches in a VTP domain. VTP advertisements can be sent over ISL, 802.1q, IEEE 802.10 and...
. By default, CDP announcements are sent every 60 seconds on interfaces that support Subnetwork Access Protocol
Subnetwork Access Protocol
The Subnetwork Access Protocol is a mechanism for multiplexing, on networks using IEEE 802.2 LLC, more protocols than can be distinguished by the 8-bit 802.2 Service Access Point fields. SNAP supports identifying protocols by Ethernet type field values; it also supports vendor-private protocol...
(SNAP) headers, including Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
, Frame Relay
Frame relay
Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...
and Asynchronous Transfer Mode
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
(ATM). Each Cisco device that supports CDP stores the information received from other devices in a table that can be viewed using the show cdp neighbors command. This table is also accessible via snmp. The CDP table information is refreshed each time an announcement is received, and the holdtime for that entry is reinitialized. The holdtime specifies the lifetime of an entry in the table - if no announcements are received from a device for a period in excess of the holdtime, the device information is discarded (default 180 seconds).
The information contained in CDP announcements varies by the type of device and the version of the operating system running on it. This information may include the operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
version, hostname
Hostname
A hostname is a label that is assigned to a device connected to a computer network and that is used to identify the device in various forms of electronic communication such as the World Wide Web, e-mail or Usenet...
, every address (i.e. IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
) from all protocol(s) configured on the port where CDP frame is sent, the port identifier from which the announcement was sent, device type and model, duplex
Duplex (telecommunications)
A duplex communication system is a system composed of two connected parties or devices that can communicate with one another in both directions. The term multiplexing is used when describing communication between more than two parties or devices....
setting, VTP
VTP
VLAN Trunking Protocol is a Cisco proprietary protocol that propagates the definition of Virtual Local Area Networks on the whole local area network. To do this, VTP carries VLAN information to all the switches in a VTP domain. VTP advertisements can be sent over ISL, 802.1q, IEEE 802.10 and...
domain, native VLAN, power draw (for Power over Ethernet
Power over Ethernet
Power over Ethernet or PoE technology describes a system to pass electrical power safely, along with data, on Ethernet cabling. The IEEE standard for PoE requires category 5 cable or higher for high power levels, but can operate with category 3 cable for low power levels...
devices), and other device specific information. The details contained in these announcements is easily extended due to the use of the type-length-value
Type-length-value
Within data communication protocols, optional information may be encoded as a type-length-value or TLV element inside of the protocol. TLV is also known as tag-length value....
(TLV) frame
Data frame
In computer networking and telecommunication, a frame is a digital data transmission unit or data packet that includes frame synchronization, i.e. a sequence of bits or symbols making it possible for the receiver to detect the beginning and end of the packet in the stream of symbols or bits...
format. See external links for a technical definition.
Hewlett-Packard
Hewlett-Packard
Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including...
removed support for transmitting CDP from HP Procurve products shipped after February 2006 and all future software upgrades. Receiving and processing CDP information is still supported. CDP support was replaced with IEEE 802.1AB Link Layer Discovery Protocol
Link Layer Discovery Protocol
The Link Layer Discovery Protocol is a vendor-neutral Link Layer protocol in the Internet Protocol Suite used by network devices for advertising their identity, capabilities, and neighbors on a IEEE 802 local area network, principally wired Ethernet...
, an IEEE standard that is implemented by multiple vendors and is functionally similar to CDP.
External links
- CDP packet definition from Cisco
- Additional CDP info from Cisco
- http://wiki.wireshark.org/CDPBreakdown and explanation of a CDP packet by WiresharkWiresharkWireshark is a free and open-source packet analyzer. It is used for network troubleshooting, analysis, software and communications protocol development, and education...
packet sniffer.]